DC/AC inverters are commonly used for various applications such as renewable power conditioning systems, electric vehicles, etc. In particular, DC/AC inverters are widely used as the second stage in two-stage renewable energy power conditioning systems. The DC/AC inverter usually operates under hard-switching conditions (i.e., neither the voltage nor the current of the power switches is zero during the switching transitions). The power semiconductors of the DC/AC inverter are switched under very high voltage at the intermediate DC-bus (usually more than 400 volts). Therefore, power semiconductors' switching losses for such inverters significantly contribute to the overall losses of the power conditioning system. In particular, the reverse recovery losses of the body diodes of the power semiconductors are inevitable for such a structure. The switching frequency of the inverter is therefore very limited (usually in the range of 10-20 kHz).
It should be noted that, for inverters, having a low switching frequency gives rise to a number of issues. Inverters with a low switching frequency require larger filters at the output to be able to inject a high quality current to the utility grid according to the strict regulatory standards. Also, a low switching frequency creates a high amount of current ripple across the inverter output inductor. This current ripple increases the core losses of the inductor as well as its high frequency copper losses. In addition, chopping the DC-bus voltage creates a significant amount of conduction and emission EMI noise, which may affect the operation of the control system and highly degrade the system reliability. Based on the above, hard-switching limits the switching frequency of the inverter and, because of this, imposes a substantial compromise in the design of the output filter and on the overall performance of the power conditioning systems.
While there are different soft-switching techniques reported in the literature, these techniques generally require many extra active/passive circuits. In particular, extra active circuits highly deteriorate the reliability of the system due to the additional complexity imposed by the active components. Also, the effectiveness of these techniques is questionable. Some studies have shown that these soft-switching techniques may add more losses to the inverter and, accordingly, greatly offset whatever advantages they may offer. Because of this, most industrial products use conventional hard-switching inverters in conjunction with a large filter to result in reliable power conditioning systems. Even though the performance of these systems is highly compromised with hard-switching and bulky lossy filters, industry prefers to use a reliable, well-known solution for the inverter.
Auxiliary circuits have been used to provide soft-switching condition for the power semiconductors of a voltage source inverter. Some soft-switching circuits use a combination of an active circuit in conjunction with passive circuits to provide soft-switching conditions. However, this approach has its drawbacks. Generally, active circuits increase the complexity of the power circuit while reducing the reliability of the systems. In addition, the losses related to the auxiliary circuits usually greatly offset the advantages of soft-switching and compromises the overall inverter performance. FIG. 1 shows a conventional auxiliary circuit used to provide soft-switching for a leg of the inverter. According to FIG. 1, usually, the auxiliary circuit includes a resonant circuit with a very high amount of peak current/voltage in conjunction with a bi-directional switch. Because of this, there are significant amount of losses which can be attributed to the auxiliary circuit. As well, there can be an added requirement that the passive components should be able to withstand the high amount current/voltage during switching transitions.
Based on the above, there is a need for a simple and practical solution which, preferably, can provide soft-switching for the power semiconductors without compromising system reliability. There is a need for solutions which mitigate if not overcome the drawbacks of the prior art.